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Alunite in the Pascua-Lama High-Sulfidation Deposit: Constraints on Alteration and Ore Deposition Using Stable Isotope Geochemistry

C. L. Deyell

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia

R. Leonardson

Barrick Gold Exploration, 293 Spruce Road, Elko, Nevada 89801

R. O. Rye

U.S. Geological Survey, Mail Stop 963, Denver Federal Center, Denver Colorado 80225

J. F. H. Thompson

Teck-Cominco, 200 Burrard Street, Vancouver, British Columbia, Canada V6C 3L9

T. Bissig

Universidad Católica del Norte, Depto. Ciencias Geológicas, Av. Angamos 0610, Antofagasta, Chile

D. R. Cooke

Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania, 7001, Australia

View larger version (16K): [in a new window]   FIG. 1. Location of the El Indio-Pascua belt relative to other major mineral districts in the south-central Andes (modified from Bissig et al., 2002) and the flat subduction zone (Barazangi and Isacks, 1976).

View larger version (38K): [in a new window]   FIG. 2. Simplified geology and major faults of the El Indio-Pascua belt, showing locations of major ore deposits in the region. Geologic information is taken from Martin et al. (1995) and Ramos et al. (1989), as summarized in Bissig et al. (2002). The upper Miocene Pascua Formation is not shown due to its restricted occurrence (see text). BdTF = Baños del Toro fault.

View larger version (39K): [in a new window]   FIG. 3. Generalized geology of the Pascua-Lama deposit (based on regional mapping by Barrick geologists and data presented in Bissig et al. (2001) and Chouinard (2003); see Figure 2 for location. Mesozoic granitic rocks (the Pascua-Lama Complex) include several bodies of similar composition, ranging from porphyritic granite to crowded granite porphyry. Several generations of Tertiary hydrothermal breccias have been grouped together and include Brecha Central, the largest breccia body in the region. Also shown are the trace of the Alex Tunnel (4,680 m asl) and line of section CA-00 (section A-A') illustrated in Figure 5. Locations of 40Ar/39Ar samples described in the text and Table 2 are given (projected to surface; abbreviations as given in Table 1).

View larger version (115K): [in a new window]   FIG. 4. Examples of alunite-pyrite-enargite mineralization. A. Banded veins in the stockwork zone surrounding Brecha Central. Alunite (white bands) alternates with pyrite-enargite (dark bands). B. Backscattered electron image showing alunite intergrown with pyrite and enargite. The latter hosts inclusions of calaverite (AuTe2). DDH-111, 200.5m.

View larger version (57K): [in a new window]   FIG. 5. Pascua-Lama property map showing the simplified distribution of alteration zones at surface (using unpub. data from D. Heberlein, 1999).

View larger version (49K): [in a new window]   FIG. 6. Section CA-00 (see Fig. 3 for location). A. Distribution of major lithological units. B. Distribution of alteration assemblages and Au-grade contours. C. Distribution of phyllosilicates (kaolinite, dickite, pyrophyllite) within the advanced argillic alteration assemblage. Mineral abbreviations: alun = alunite, dick = dickite, ill = illite, jar = jarosite, kao = kaolinite, pyrophyllite (pyl), quartz (qtz).

View larger version (161K): [in a new window]   FIG. 7. Photographs of alteration minerals. A. Backscattered electron image showing irregular alumino-phospho-sulfate grains (bright zones) in cores of alunite associated with advanced argillic alteration (alun). DDH-116, 289m. B. Steam-heated alteration in drill core sample DDH 119-47m above the Frontera zone. Alunite + quartz replaces feldspar phenocrysts. C. Backscattered electron image of ore-stage alunite containing REE-rich alumino-phospho-sulfate (APS) inclusions (florencite). Sampled from DDH-111, 189.9m. D. Backscattered electron image showing oscillatory PO4 ± Sr-enriched bands (light bands) in coarse-grained magmatic steam alunite (sample PS-26c). E. Backscattered electron image of pseudocubic alunite (gray) with overgrowing jarosite (white) in late-stage vein. F. Backscattered electron image showing supergene alunite, jarosite, and intermediate alunite-jarosite solid solution. Sample PM-33, Alex Tunnel.

View larger version (23K): [in a new window]   FIG. 8. Range of 40Ar/39Ar age data (with 2 error bars) for alunite from the alunite-pyrite-enargite (APE)-stage ore, steam-heated alteration (SH), magmatic steam (MS), late-stage veins (LV), and one sample of supergene jarosite (SP). Also indicated is the approximate minimum age for preore advanced argillic alteration (AA1), based on age dates for advanced argillic alteration in the Frontera and Lama areas from Bissig et al. (2001). The age of the Pascua Formation rhyodacite dike is also reported in Bissig et al. (2001). The cross-hatched area represents the possible age range of APE mineralization, given constraints from preore AA1 alteration and postore MS alunite.

View larger version (15K): [in a new window]   FIG. 9. Range of 34S values (in ) for all stages of Pascua alunite. Also shown are data for associated sulfides, jarosite, and barite. The estimated 34SS ~ 2 to 4 per mil is indicated by the shaded region. Range of 34Salun-py temperatures for preore advanced argillic (AA1) alteration and alunite-pyrite-enargite (APE) mineralization are given, as well as estimated H2S/SO2 ratios (based on 34S values for coexisitng pyrite and alunite). See text for discussion.

View larger version (20K): [in a new window]   FIG. 10. D, 18OSO4, and 18OOH values for alunite in the preore advanced argillic assemblage (AA1). Fluid compositions (DH2O and 18OH2O) in equilibrium with alunite are calculated from equations of Stoffregen et al. (1994) at 200° to 380°C (based on range of 34Salun-py for AA1 alteration). Reference lines and fields shown include: meteoric water line (Craig, 1961), kaolinite line (Savin and Epstein, 1970), typical fluids dissolved in felsic magmas (Taylor, 1988), the range of water compositions discharged from high-temperature fumaroles (volcanic vapor; Giggenbach, 1992), and the composition of paleometeoric waters in the El Indio-Pascua belt (estimated from paleotopography; B. Taylor, pers. commun., 2001).

View larger version (25K): [in a new window]   FIG. 11. D, 18OSO4, and 18OOH values for alunite from alunite-pyrite-enargite (APE)-stage ore and steam-heated (SH) alunite. The range of calculated DH2O and 18OH2O for fluids in equilibrium with APE and SH alunite (calculated from equations of Stoffregen et al., 1994) are shown as separate fields. Temperatures used for calculations (SH = 60°–140°C; APE = 200°–340°C) are derived from 34Salun-py and 18OSO4-OH isotope data. Reference lines and fields as given in Figure 9.

View larger version (42K): [in a new window]   FIG. 12. D, 18O SO4, and 18OOH values for magmatic steam alunite (MS), late vein alunite (LV), and jarosite (jar). The range of DH2O and 18OH2O for fluids in equilibrium with each sample (bars and oval) are calculated from equations of Stoffregen et al. (1994) and Rye and Stoffregen (1995). Temperatures used for calculations (LV = 50°–100°C; Jar = 25°–50°C) are derived from 18OSO4-OH data. The range of temperatures for MS alunite (150°–200°C) is based on estimates of depositional temperatures related to magmatic steam processes (Rye et al., 1992; Rye, 1993). Reference lines and fields as given in Figure 10 together with the additional supergene alunite sulfate field (SASF) from Rye et al. (1992), and the supergene jarosite OH zone (SJOZ) and supergene jarosite sulfate field (SJSF) as described in Rye and Alpers (1997).

View larger version (35K): [in a new window]   FIG. 13. Schematic section showing the evolution of the Pascua deposit and associated alteration assemblages over time. A. ~9.1 to 8.8 Ma: widespread, early hydrothermal alteration in the greater Pascua region created patchy vuggy silica (silicic) alteration zones and widespread advanced argillic assemblages (AA1) grading outward to argillic and propylitic (not shown) assemblages. Sericite at depth is inferred from old core logs and company reports. Minor steam-heated alteration zones may have developed at or near surface at this time. B. 8.8 to ~8.4 Ma: main-stage Au-Ag-Cu mineralization followed brecciation in the Brecha Central area and in several smaller satellite breccia bodies. Alunite-pyrite-enargite (APE) mineralization was deposited in open spaces in the breccia matrices and surrounding vein networks. Upwelling magmatic fluids (condensed magmatic vapors) overwhelmed and displaced meteoric water. Ore deposition was coincident with lowering of the water table due to regional uplift and erosion. C. 8.4 to 7.9 Ma: postore alteration and late-stage processes coincided with the waning stages of the magmatic-hydrothermal system. Pulses of magmatic vapor deposited magmatic steam alunite near surface above Brecha Central. Late-stage alunite ± jarosite formed thin veinlets and disseminations from mixed magmatic-meteoric fluids down to depths of ~600 m below the present-day surface. Isolated disseminations and veinlets of supergene jarosite ± alunite (not shown) formed from cooler meteoric waters that penetrated back into the system.

View larger version (42K): [in a new window]   FIG. 14. Log (O2)-pH diagram illustrating condition in the Pascua alunite-pyrite-enargite (APE) mineralizing event at 275°C and vapor saturation (59 bars). The stability fields of alunite (alun), enargite (en), hematite (hem), magnetite (mag), muscovite (musc), pyrrhotite (po), pyrite (py), tennantite (tenn), and the predominance fields of aqueous sulfur-bearing species are shown. Shaded and stippled gray areas represent total gold solubility contours for Au(HS)–2, AuHS(aq), and AuCl–2 at 1 ppb, 10 ppb, and 1 ppm, resepectively. Calculations for the distribution of species and gold concentrations are modified after Cooke et al. (1996). The white area labeled "APE ore fluids" and the black area contained within represent the chemical characteristics of alunite-pyrite-enargite mineralizing fluids. The black area represents the bulk of alunite-pyrite-enargite ore and is contained within the stability fields of alunite, pyrite, and enargite and is constrained by H2S/SO4 ratios from 1 to 2.5 (based on 34S results from this study). Local mineralogical variability (e.g., monomineralic veins of alunite, enargite, and/or pyrite and the presence of native sulfur) suggests that the composition of the ore fluid varied over a wider range locally, schematically represented by the larger white area.

View larger version (14K): [in a new window]   FIG. A1. Argon release patterns for different stages of alunite and jarosite from Pascua. The stippled line indicates the steps included in the plateau age for each sample. A. Sample P05, alunite from the alunite-pyrite-enargite (APE) ore stage; plateau age = 8.78 ± 0.63 Ma. B. Sample P17, alunite from late-stage veins; plateau age = 7.97 ± 1.59 Ma. C. Sample P18, steam-heated alunite; plateau age = 9.14 ± 1.98 Ma. D. Sample Pj31, supergene jarosite; plateau age = 7.98 ± 0.43 Ma.